Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), and incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
A Stirling cycle engine operates under pressurized conditions. Stirling engines contain a high-pressure working fluid, preferably helium, nitrogen or a mixture of gases at 20 to 140 atmospheres pressure. A Stirling engine may contain two separate volumes of gases, a working gas volume containing the working fluid, called a work-space or working space, and a crankcase gas volume, the gas volumes separated by piston seal rings. The crankcase encloses and shields the moving portions of the engine as well as maintains the pressurized conditions under which the Stirling engine operates (and as such acts as a cold-end pressure vessel). A pressurized crankcase removes the need for high pressure sliding seals to contain the work-space working fluid and halves the load on the drive component for a given peak-to-peak work-space pressure, as the work-space pressure oscillates about the mean crankcase pressure. The power output of the engine is proportional to the peak-to-peak work-space pressure while the load on the drive elements is proportional to the difference between the work-space and the crankcase pressures. FIG. 1 shows typical pressures in the gas volumes for such an engine.
The action of the piston rings can raise or lower the mean working pressure above or below the crankcase pressure, substantially mitigating the above-mentioned advantages of a pressurized crankcase. For example, manufacturing marks, deviations and molding details of the rings can produce preferential gas flow in one direction between the work-space and the crankcase. The resulting difference in pressure between the work-space and the crankcase can produce as much as double the load on engine, while peak-to-peak pressure and thus engine power increases only fractionally (see, e.g., FIG. 2). In summary, pumping up the workspace mean pressure significantly increases engine wear with only a small attendant increase in power production.